Devices such as cut-outs and switching devices have been used in conjunction with incandescent and other light bulbs for many years. Some of these systems have been for the purpose of maintaining an electrical connection through a failed light bulb, in order to maintain electrical current to other series-connected bulbs in a grid. U.S. Pat. No. 466,400 discloses a spring loaded mechanism to re-connect the conductors in an incandescent bulb when the filament has been broken, for this purpose.
U.S. Pat. No. 476,530 discloses a switch for the same purpose, in which plates separated by a small distance are charged by the current-carrying elements to which they are attached. When the filament breaks, the charge between these plates is increased such that the plates are sufficiently attracted to each other that they come into contact, thereby restoring current to the remaining series-connected grid.
U.S. Pat. No. 3,794,880 discloses a bulb having a wire connecting the current-carrying elements, which is insulated therefrom by a metal oxide layer during normal operation. When the filament is broken, the resulting potential between the connecting wire and the elements is sufficient to bridge the insulation layer, thereby restoring current to the remaining bulbs in a series-connected grid.
Other known switching devices are designed to cut the flow of current to the inner light-generating member of a high intensity discharge (HID) lamp, after the outer bulb is broken, in order to avoid danger from the intense ultraviolet (UV) rays emitted from the inner member. In such HID lamps, the UV emissions are normally either filtered out or converted to visible light by the intact outer member. Thus, U.S. Pat. No. 4,032,816 discloses a HID lamp having a spring-loaded safety switch, in which the spring bears against the outer bulb to hold the switch in a normally-closed position, thereby allowing current to flow to the inner light-generating member. When the outer bulb is broken, the spring-loaded switch is opened, cutting current to the inner member. U.S. Pat. No. 4,752,718 discloses a similar spring-loaded safety switch for a HID bulb.
None of the prior devices work to prevent current from escaping from a damaged lamp element into the immediate surroundings, particularly in a situation where escape of such current could instantly cause severe or even fatal injury to people in the immediate vicinity of the failed bulb.
Of particular concern are swimming pool lights in which an incandescent bulb is enclosed within a sealed envelope having a glass wall through which light is directed into the pool water. The light fixture is permanently embedded in a side wall of a swimming pool. If the glass envelope is broken, the bulb itself will shatter due to the in-rushing cold water striking the hot glass. It is clear that the possibility of electrical shock exists from consequent contact of the current and the water, making it imperative that the current be diverted or shut off immediately. Even though the current supplied to such lights may be less than 15 volts, a significant danger of electrical shock exists when such a fixture is broken. In the art, this has been accomplished by a variety of circuit breakers, ground-fault current-interrupters, and spring-loaded devices mounted external to the bulb or lamp. An example is a grounding connection which is pre-loaded to swing in to the area vacated by the shattered envelope, the current connection being thereby diverted or grounded.